Externally powered electric vehicles are commonly used in many industries, particularly the mining industry, in which safety requirements prevent or restrict the underground use of internal combustion engine-powered vehicles. Such electric vehicles typically draw power from an external trolley line or trailing umbilical cord that supplies alternating current (AC). Trolley lines are typically employed by hauling trucks; umbilical lines are generally used by scoop vehicles. The alternating current may be used to power an AC motor or may be rectified to power a direct current (DC) powered motor.
Existing vehicles using AC motors have had several disadvantages relating to their controllability. Previously existing AC controllers were unavailable or unsuitable for use on mobile vehicles due to their size, complexity, and environmental sensitivity, particularly to acceleration and mechanical shock. As a result, such systems have been used only for monorail-type storage and retrieval systems having limited mobility within a restricted track such as along a single aisle in an automated warehouse. In such systems, the controller is not transported on the car, and the environment is easily controlled.
At low speeds, AC motors are susceptible to "cogging," which impairs smooth operation. AC motors have been employed in electric vehicles, but these have been constant speed motors. Thus, they require a complex transmission and a clutch slipping technique employed by skilled operators to drive the vehicle at a variable speed.
DC motors, on the other hand, are easily controllable. However, as a practical matter they require an inefficient conversion of AC power to DC. Such an AC to DC conversion system is shown, for example, in U.S. Pat. No. 4,483,148 to Minami.
In addition, DC motors are more complex than AC motors, with brushes that are prone to wear. As a result, they are more expensive to maintain and less reliable than AC motors. The trolley line approach is also unsuitable for providing DC power from an external source, due to power loss through line resistance, particularly on long lines. Some prior art systems employ both AC and DC motors to gain the advantages of each system. However, this comes at the cost of added complexity and redundancy. U.S. Pat. No. 4,099,589 to Williams, for example, shows an electric car with AC and DC drive motors. The car may be driven by the DC motor alone during short range stop-and-go driving where controllability is important, and may be driven by the AC motor alone during relatively constant speed long range driving when efficiency is critical. The use of redundant motors, however, substantially increases cost, and both motors rely on an on-board internal combustion engine for electric power generation.